Name: Holmberg II = UGC 4305 = Arp 268
RA: 08 19 05
Dec: +70 43 12 (J2000)
Constellation: Ursa Major
Size: ~ 7' x 5'
Mag: 11.4
SBr: ~15 mag/arcmin² (23.9 MPSAS)
Holmberg II is a faint irregular dwarf galaxy in Ursa Major, about 10 Mly away from us. The galaxy was first described by Swedish astronomer Erik Holmberg of Lund Observatory in his 1950 "Holmberg dwarfs" paper [1]. It is a member of the nearby M81--NGC 2403 group, likely the closest one to us in that group [3].
POSSIIRed.jpg
[DSS POSSII Red plate. Credit: MAST/STScI]
In the professional world, Holmberg II seems to be rather well-studied mostly for its gas-rich interstellar medium [2], star formation [3,4], and an ultra-luminous X-ray source [5]. Here is a beautiful HST image of the HII regions in this galaxy:
1567216781905-heic1114a_410.jpg
[Credit: HST/NASA/ESA]
In the amateur world, Holmberg II is perhaps best known for being part of a "short" observing program of the 9 Holmberg dwarf galaxies [6], and also of the Arp catalog.
I first observed this object in 2013 with my 18" from Bortle 2 skies near Pontotoc, TX. At 200x, I was able to identify mottling and two brighter regions marked in this rough sketch:
HoII_2013_11_02.png
My meager notes from an observation in Dec 2015 from the darker Commanche Springs Astronomy Campus in Crowell, TX say that the object was rather easy with my 18", and I was able to pick out the central bright region of the object and also hold two brightenings within that region. The HII regions could not be identified. The notes further say that it was also visible in my friend's 5" Takahashi refractor, and while it was certainly not easy to see, it was not as indistinct as I had presumed it would be.
I would love to give this object another shot to see if I can see the HII knots. Hodge, Strobel and Kennicutt [7] describe 82 HII regions in this galaxy! [7] and [3] also point out that an unusually large number of HII regions in this galaxy show ring structures. Perhaps the brightest of these HII regions is cataloged as Holmberg II [HK82] 11 in NED (SIMBAD has it listed as HK Ho II 11) and appears to be well within the range of our amateur telescopes. Here is the position data for this HII knot:
RA: 08 19 13
Dec: +70 43 08 (J2000)
This is the HII region seen immediately below the center in the HST image posted above.
Finally, let me end with some biographical facts about Prof. Holmberg [8,9]:
- Holmberg was one of the first to consider determining galaxy masses from rotation curves.
- He discovered the Holmberg effect – significantly more number of satellite galaxies lie in projection above the poles of spiral galaxies than in the equatorial planes.
- He is famous for his work on interacting galaxies. In 1941, he constructed an analog computer with light bulbs and photocells to simulate the collisions of galaxies!
- His waltz with Russian astronomer Anna Massevitch was one of the highlights of the closing banquet of the IAU General Assembly in Brighton in 1970!
As always,
"Give it a go, let us know"!
References:
[1] http://adsabs.harvard.edu/pdf/1950MeLuS.128....5H
[2] http://adsabs.harvard.edu/pdf/1992AJ....103.1841P
[3] https://iopscience.iop.org/article/10.1086/308241/pdf
[4] https://iopscience.iop.org/article/1...57/aab041/meta
[5] https://phys.org/news/2015-06-unexpe...s-rapidly.html
[6] https://www.springer.com/gp/book/9781852337520
PS: High-res HST image to enjoy! https://archive.stsci.edu/cgi-bin/hl...)%20UGC-4305-2
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